1. Technical Field of Invention
The present invention relates to a piston ring for use in internal combustion engines.
2. Description of Related Art
In recent years, engines must meet increased demands for higher output, high revolutions per minute and longer product service life and must also comply with ever stricter exhaust gas emission regulations. The piston ring must therefore function in a yet harsher operating environment. Many of such piston rings are formed typically with a hard surface treatment layer such as chromium plating film, nitrided layer or physical vapor deposition film on the upper and lower surfaces and the outer circumferential surface.
However, piston rings having a nitrided layer or physical vapor deposition film are prone to wear the ring grooves of the aluminum alloy pistons. In particular, piston rings having nitrided layers are prone to cause aluminum adhesion on the upper and lower surfaces, increasing the wear on the ring grooves.
In the initial stage of engine operation, break-in between the cylinder and piston ring is not sufficient so that scuffing may occur between the cylinder and piston ring formed with the above mentioned hard surface treatment layer. Accordingly, a method to improve initial break-in characteristics of these hard surface treatment layers or a hard surface treatment layer having both scuffing resistance and wear resistance is needed.
To respond the above problem, the following arts have been proposed.
1. Japanese Patent Laid-open No. 8-184375
A film of either 98.0 to 99.5 percent molybdenum or chromium or both and the remainder of nitrogen was formed by physical vapor deposition on a nitrided layer or physical vapor deposition film in order to improve the initial break-in characteristics.
2. Japanese Patent Publication No. 5-54594
A TiN film was formed over a hard film comprised of CBN, TiC, Ti (C,N), SiC, diamonds or Al.sub.2 O.sub.3 in order to improve the scuffing resistance.
On the other hand, the following art involved coating of artificial diamond material has been proposed.
3. Japanese Patent Laid-open No. 3-260362
A thin film of artificial diamond material was formed on the piston ring and on the top surface, external circumferential surface and ring groove of the piston in order to improve durability.
Further, the following art involved use of a diamond-like carbon film has been proposed.
4. Japanese Patent Laid-open No. 5-179451
In order to restrict the adhesion of ferrite structures, a film having amorphous carbon as the main constituent in which tungsten and/or silicon was dispersed was formed on the sliding surface which mates with a sliding surface formed of iron-type material containing ferrite. This technology is utilized for instance, in the hydraulic valve in power steering mechanisms.
However, under harsh engine operating conditions, the technology proposed in the above first method has the drawback that the molybdenum or chromium film wears away quickly. The technology proposed in the above second method has a TiN film with excessive hardness so that resistance to scuffing is inadequate. The technology proposed in the above third method has no detailed description of any kind regarding the thin film of artificial diamond material. The technology proposed in the above fourth method provides no description of aluminum adhesion on the upper and lower surfaces of the piston rings mating with aluminum alloy pistons. Besides the above mentioned technologies, a method is known, for suppressing aluminum adhesion in the initial stages of engine operation by coating the upper and lower surfaces of the piston ring with a synthetic resin film containing solid lubricant. However, this synthetic resin film has poor wear resistance, so this method has the drawback that suppression of the aluminum adhesion does not last long.